Downhole drills are used for oil drilling, geothermal drilling, and other deep earth penetration applications. Downhole drills include rotary and percussive drills. For nearly any drilling method, rotational energy must be transferred downhole in order to promote rock reduction. The drill bit may be rotated by an electric motor or fluid/hydraulic system. The rotating action can be produced either at the surface or near the drill bit. In addition to rotational cutting, drills may also be pressurized or mechanically actuated to force the drill bit to hammer against the rock/earth. Prior art rotation systems and methods are complex, require large form factors to create sufficient torque, and require a high degree of maintenance.
The most common method of downhole energy transfer is rigid drill pipe. The drill pipe is rotated from the surface, with drilling joints added for tripping (moving in and out of the hole). For this type of system, the entire drill string rotates. Typically a rotary table system or a top drive is used to drive the drill string. Although it is well suited for vertical drilling, it has limited applications in directional drilling because the drill string curvature and thrust loads generate additional torque that the surface based motor must overcome and drill pipe survive.
Downhole techniques used to generate rotation such as positive displacement motors (PDMs) are limited in their temperature range due to the use of elastomers. Energy resources like geothermal and deep oil and gas wells lie in hot (160° C.−300° C.), and often hard rock. The high-temperatures limit the use of PDM's in those environments. In addition, PDMs generate rotation by eccentric motion of the rotor around the motor case which induces significant lateral vibration to the drilling assembly.
What is needed is a drill rotation system and method that overcomes the limitations of the prior art.